Remove unnecessary top()->document() null check from PluginListChanged.

This was introduced in r135340 in an attempt to fix a crash in issue
125969, which it ended up not fixing, but nobody reverted it
afterward.  It doesn't appear to be necessary.

BUG=466297

Review-Url: https://codereview.chromium.org/2145353004
Cr-Commit-Position: refs/heads/master@{#406005}

chrome/renderer/plugins/chrome_plugin_placeholder.cc

@@ -260,9 +260,6 @@ void ChromePluginPlaceholder::OnCancelledDownloadingPlugin() {
 void ChromePluginPlaceholder::PluginListChanged() {
   if (!GetFrame() || !plugin())
     return;
-  blink::WebDocument document = GetFrame()->top()->document();
-  if (document.isNull())
-    return;
 
   ChromeViewHostMsg_GetPluginInfo_Output output;
   std::string mime_type(GetPluginParams().mimeType.utf8());